Silicon, especially high-resistivity silicon, is widely used in terahertz (THz) components due to its broadband transparent window spanning from microwave to mid-infrared with little absorption of electromagnetic waves. Its range of application in THz spectroscopy spans from windows and lenses to filters and beam-splitters. However, because of its inherent high dielectric constant, silicon is usually associated with high Fresnel reflection loss (30% reflectivity in THz power from a single surface) and possibly limiting spectral resolution stemming from the finite time window as a result of strong secondary reflection from its surfaces.
Previously, anti-reflection (AR) coatings for silicon have been implemented in several ways:                1) One method was to use quarter-wave thin film as AR layer which had a refractive index of n=√{square root over (nsilicon)}. Unfortunately, this method was only suited to enhance transmission of a single frequency, and is inadequate for broadband THz time-domain spectroscopy.        2) Silicon nano-tip was reported as another AR coating method, whose improvement of transmission was unfortunately limited to frequencies higher than 1 THz.        3) A photonic crystal slab made with air holes in silicon was illustrated as another option to enhance transmission from 0.1 THz to 0.45 THz, but, unfortunately, the AR effect was narrowband with performance at higher frequencies dramatically deteriorated.        4) Multi layer coating has been used for broadband anti-reflection. Unfortunately, the design and fabrication process is complicated with a multi-layer coating method due to a lack of materials which have low absorption and suitable refractive index between that of silicon and air at THz frequency.        
Therefore, there is a need for a tunable broadband anti-reflection apparatus having improved broadband anti-reflection functions at terahertz (THz) frequencies, which may be manufactured and tuned through a practical process.